It is well known in the art that magnetic sensors can be employed in position and speed sensors with respect to moving ferromagnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetic sensor is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative, and in close proximity, to the magnetic sensor, such as a toothed wheel, produces a varying magnetic flux density through the magnetic sensor.
FIG. 1A is an example of a magnetic sensor 50 according to the prior art, wherein the magnetic sensor element 10 is mounted on the bottom surface 12 of a permanent magnet (bias magnet) 14 magnetized in a direction 16 perpendicular to the direction of motion 18 of target wheel 20 having teeth 22 and slots 24. The total package thickness 26 is determined by the thickness 30 of the magnetic sensor element 10, incorporating protection and electrical connections for the magnetic sensor element, and the magnet length 32. The magnet length 32 cannot be small since it determines the magnetic signal strength detected by magnetic sensor element 10 and a total package thickness 26 of 5 millimeters or more is common.
The resolution of magnetic sensor 50 is related to the number of teeth 22 of target wheel 20. In some cases, the number of teeth 22 is fixed by external constraints, for instance, when target wheel 20 is a gear used for both mechanical advantage and for position sensing. The number of teeth 22, in such a case, may not be sufficient to provide the desired resolution.
FIG. 1B is a plot 52 of the magnetic flux density detected by the magnetic sensor 50 according to the prior art of FIG. 1A as the target wheel 20 passes the magnetic sensor. The larger magnetic flux density 22′ represents the passage of a tooth 22 past the sensor 50 whereas the smaller magnetic flux density 24′ represents the passage of a slot 24 past the sensor.
Accordingly, what is needed in the art is a more robust magnetic sensor configuration enabling a smaller total package thickness and a means for increasing the resolution of the magnetic sensor.